robert:
From Art Trevs, add support for saving external shader files.
From Robert …

Property svn:mergeinfo set to
/OpenSceneGraph/branches/OpenSceneGraph-2.8.2/src/osgDB:10664/OpenSceneGraph/trunk/src/osgDB:9769,9879-9880,9895,9932,10010,10150,10208,10340,10417,10456,10487,10622-10623,10625,10671-10672,10697,10722,10753,10758,10788,10818,10854-10855,10858,10887,10891,10923,10933,11019,11032,11034-11035,11111,11127,11131,11175